Field of the Invention
The present invention relates to an image forming apparatus by using an electrophotographic system.
Description of the Related Art
In an image forming apparatus that forms an image on a recording material using an electrophotographic system such as a copier, a printer, and a facsimile machine, a configuration including a developing apparatus for visualizing an electrostatic latent image with nonmagnetic one-component toner has been known. As such a developing apparatus, there has been known one including a developing roller serving as a developer bearing member that bears and transports toner and a supply roller serving as a developer supply member that is arranged around the developing roller and supplies the toner to the developing roller. In the developing apparatus, the toner is supplied to the developing roller while being friction-charged by the mechanical rubbing between the supply roller and the developing roller. The supplied toner is controlled to have a certain thickness on the developing roller by a developer control member, and then transported to a developing region representing the adjacent region between the developing roller and a photosensitive drum serving as an electrostatic latent image bearing member to visualize an electrostatic latent image as a toner image.
Residual toner on the developing roller (hereinafter called “development residual toner”) that has not been used for developing in the developing region is scraped from the developing roller by the mechanical rubbing between the supply roller and the developing roller at the contact portion between the developing roller and the supply roller. At the same time, the toner is supplied from the supply roller to the developing roller. On the other hand, the scraped toner is mixed with toner inside and around the supply roller. Moreover, there has been generally used a method for applying a bias for generating the potential difference between a developing roller and a supply roller to supply toner from the supply roller to the developing roller and collect the toner from the developing roller with an electrostatic force (Japanese Patent Application Laid-open No. H9-15976). In Japanese Patent Application Laid-open No. H9-15976, there has been proposed a method for performing control to apply a bias for collecting the toner on an intermediate roller corresponding the developing roller during an image forming period and apply a bias for forming a toner layer on the intermediate roller during a non-image forming period.
Meanwhile, for an image formed by a series of image forming operations, an image and density intended by a user need to be output. In addition, in a full-color image generated by a plurality of image forming stations, the reproducibility of a tinge becomes important. Therefore, for the purpose of increasing the selection range of a tinge, there has been generally used a method for changing the rotation speed of a developing roller to change the peripheral velocity ratio of the developing roller with respect to a photosensitive drum (Japanese Patent Application Laid-open No. H8-227222). Hereinafter, an image forming operation in which the amount of supplied toner per unit area from a developing roller to a photosensitive drum is increased to increase density or color gamut will be called “high-density mode.”